PURPOSE: To provide an improved system for controlling the differential pressure between a laboratory having many internally arranged fume hoods and a standard space which is to be positioned desirably adjacent to the laboratory room.

CONSTITUTION: The differential pressure control system maintains the pressure in a room, such as laboratory having one or more fume hoods at a prescribed pressure level to the pressure in a standard space, such as the adjacent passage way, adjacent room, etc., in a building. The system contains a room controller 22 constituting part of the heating, ventilating, and air-conditioning system of the building. The room controller 22 can receive an electric signal proportional to the quantity of air exhausted through each fume hood from each fume hood controller 20. According to a air quantity indicating signal transmitted to the controller 22 from each controller 20, the differential pressure control system adjusts the quantity of air supplied to the room. Therefore, the differential pressure between the pressure in the room and that in the standard space can be maintained at a desired level in a relatively short response time.